3U0 Calculation

[mLp85]

Hi,

Looking for some help in calculating the 3U0 voltage during single phase to earth fault. I have limited information to begin with and I can't use modelling software as the source is an inverter so the algorithm will not converge .

I have:

- Nominal L-L Voltage
- Line impedance between source and fault
- Fault current

I'm reluctant to arbitrarily set the ResV protection without first confirming that it will be sensitive to faults at the end of the line.

 

[davidbeach]

What does the inverter do during the fault? There's no single answer to that question.

What else might be between the inverter and the fault, like a transformer perhaps? What winding configuration?

In other words, it depends.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[mLp85]

The inverter supplies the grid via a step up transformer and line.

I can't determine the inverters behaviour during the fault, as there is no 'standard' for short circuit calculation when the contribution is purely from the IBR. We assume the anti-islanding protection will kick in and disconnect the circuit but given that the there is no zero sequence pathway through the transformer (star/delta) with the delta being unearthed and on the grid side, exactly how fast it does this is the big unknown.

So given that the voltage network should perform much the same as a synchronous generator, I thought we could simply treat the inverter as an 'infinite bus' and consider the transformer as the source impedance, then add the line impedance and vary values of fault resistance to see the range of 3U0's possible for the scenario.

Any thoughts on this?

 

[Parchie]

There is supposed to be zero residual voltage on the broken-delta points in your GPT if the system is healthy (3U0 = UA0+UB0+UC0 = 0). Just set 3U0 at 5V and I think you're fine. If you don't want it, you can disable 3U0 protection by setting it to infinite (no tripping)